The present invention relates to digital printing and especially to a method and device useful for liquid electrophotography. Specifically, the present invention is of a composition that allows electrophotographic printing with improved scratch resistance.
Digital printing involves technologies in which a printed image is created directly from digital data, for example using electronic layout and/or desktop publishing programs. Known methods of digital printing include full-color inkjet, electrophotographic printing, laser photo printing, and thermal transfer printing methods.
Electrophotographic printing techniques involve the formation of a latent image on a photoconductor surface mounted on an imaging plate. The photoconductor is first sensitized to light, usually by charging with a corona discharge, and then exposed to light projected through a positive film of the document to be reproduced, resulting in dissipation of the charge in the areas exposed to light. The latent image is subsequently developed into a full image by the attraction of oppositely charged toner particles to the charge remaining on the unexposed areas. The developed image is transferred from the photoconductor to a rubber offset blanket, from which it is transferred to a substrate, such as paper, plastic or other suitable material, by heat or pressure or a combination of both to produce the printed final image.
The latent image is developed using, either a dry toner (substantially toner particles mixed with a powder carrier) or a liquid toner (substantially a suspension of toner particles in a liquid carrier). The toner particles generally adhere to the substrate surface with little penetration into the substrate. The quality of the final image is largely related to the size of the toner particles, with higher resolution provided by smaller toner particles.
Dry toners used in solid electrophotography are fine powders with a relatively narrow particle size distribution that are expelled from fine apertures in a print head. A typical dry toner is predominantly composed of a heat-sensitive polymer (e.g., acrylic, styrene) and a pigment such as carbon black with a solid carrier, typically resin coated iron or steel powders. Variations in particle shape and charge-to-mass ratio as well as dust particles found in dry toner may cause technical difficulties during the printing process. Larger or irregularly shaped particles can cause blockage while dust particles that are too small to hold a sufficient charge to be controllable adhere to the print head surface.
Liquid toners used in liquid electrophotography are composed of pigmented or dyed thermoplastic resin particles suspended in a non-conducting liquid carrier, generally a saturated hydrocarbon. Offset-preventing and release-facilitating oil, such as silicone oil, is often used to increase the efficiency of toner transfer from the imaging surface. The liquid toner is electrophotographically charged and brought into contact with the photoconductor surface to develop the latent image. When transferred to an offset blanket and heated, the particles melt and fuse to form a tacky polymer film. When the tacky polymer film comes in contact with a cooler surface, such as a paper substrate, the film hardens and adheres to the substate, and peels away from the blanket, laminating the paper. The ink is deposited onto the substrate essentially dry, and desired print finishing can be performed immediately. Since the ink is transferred completely from the blanket to the substrate, a new layer in a different colour can be created for every rotation of the press.
Electroink®, commercially available from Hewlett Packard Company (Maastricht, The Netherlands), is a particularly effective liquid toner for electrophotography containing a dispersion of pigmented polymer particles ranging in size from 1-2 microns, in a hydrocarbon carrier, such as an aliphatic hydrocarbon. A preferred carrier used in the preparation of Electroink® is available under the tradename Isopar® from Exxon Mobil Corporation, Fairfax, Va., USA.
The small particle size used in liquid toners such as Electroink® allows the printing of high resolution, high gloss images, with sharp edges and very thin image layers. However, due to the fact that the print produced by liquid toner is not absorbed into the paper but is attached to the paper surface by adhesive traction, the print may be prone to damage by scratching, peeling or flaking.
International Patent Application No. PCT/US2005/026627 of the Applicant which is incorporated by reference as if fully set forth herein teaches a UV-curable composition that allows electrophotographic printing having improved wear resistance, especially peeling and flaking resistance. A specific composition taught in International Patent Application No. PCT/US2005/026627, including a mixture of monoacrylates and diacrylate provides a significant improvement to the art of printing. Despite the fact that the specific composition of International Patent Application No. PCT/US2005/026627 provides print having excellent peeling and flaking resistance, there is a desire for print having even better scratch resistance.
It would be highly advantageous to provide print produced by electrophotographic printing having improved scratch resistance.